fix(worker): kill cancelled runs' processes and make MCP approve actually merge

- CancelAsync now signals the running Claude process of the cancelled task and
  its cascaded children via the new RunCancellationRegistry (queue + override
  slots register their CTS there) instead of only flipping DB state.
- external MCP review_task 'approve' now mirrors the hub's ApproveReview:
  unit merge for parents, ApproveAndMergeAsync for childless tasks, optional
  targetBranch; ReviewTaskResult carries mergeStatus/conflicts.

tests/ClaudeDo.Worker.Tests/WaitingForChildrenLifecycleTests.cs

@@ -214,6 +214,41 @@ public sealed class WaitingForChildrenLifecycleTests : IDisposable
         Assert.Null(queuedChild.BlockedByTaskId);
     }
 
+    [Fact]
+    public async Task Cancelling_WaitingForChildren_parent_stops_running_child_process()
+    {
+        using (var ctx = _db.CreateContext())
+        {
+            ctx.Lists.Add(new ListEntity { Id = "l1", Name = "L", CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity { Id = "par", ListId = "l1", Title = "Parent",
+                Status = TaskStatus.WaitingForChildren, CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity { Id = "c_running", ListId = "l1", Title = "Running child",
+                Status = TaskStatus.Running, ParentTaskId = "par", CreatedAt = DateTime.UtcNow });
+            await ctx.SaveChangesAsync();
+        }
+
+        using var childCts = new CancellationTokenSource();
+        _built.RunCancels.Register("c_running", childCts);
+
+        var result = await _built.State.CancelAsync("par", DateTime.UtcNow, default);
+
+        Assert.True(result.Ok);
+        Assert.True(childCts.IsCancellationRequested);
+    }
+
+    [Fact]
+    public async Task Cancelling_running_task_signals_its_registered_run()
+    {
+        var id = await SeedRunningStandaloneAsync();
+        using var cts = new CancellationTokenSource();
+        _built.RunCancels.Register(id, cts);
+
+        var result = await _built.State.CancelAsync(id, DateTime.UtcNow, default);
+
+        Assert.True(result.Ok);
+        Assert.True(cts.IsCancellationRequested);
+    }
+
     // ─── FinalizePlanningAsync ────────────────────────────────────────────
 
     private async Task<string> SeedActivePlanningParentAsync(string id = "par")